1. Field of the Invention
The present invention relates to a control device for a hybrid electric vehicle, and more particularly to a control device for a hybrid electric vehicle capable of transmitting each of a driving force of an engine and a driving force of an electric motor to driving wheels of the vehicle.
2. Description of the Related Art
A so-called parallel type hybrid electric vehicle capable of transmitting each of a driving force of an engine and a driving force of an electric motor to driving wheels of a vehicle has been conventionally developed and come into practical use.
As such a parallel type hybrid electric car, for example, Unexamined Japanese Patent Publication No. 5-176405 (hereinafter, referred to as Document 1) has proposed a hybrid electric car in which a clutch that mechanically connects/disconnects an engine to/from an automatic transmission is provided and a rotary shaft of an electric motor is coupled between an output shaft of this clutch and an input shaft of the automatic transmission.
In such a hybrid electric car as disclosed in Document 1, a state where the clutch is engaged to enable transmitting driving forces to driving wheels from both the engine and the electric motor can be switched to/from a state where the clutch is disengaged to enable transmitting a driving force of the electric motor alone to the driving wheels.
For example, when depression on an accelerator pedal is released and the hybrid electric vehicle travels while reducing a speed in a state where a brake of the vehicle is not operated, a decelerating torque that can obtain the substantially same deceleration as that obtained when substantially the same vehicle using an engine alone as a power source performs the same deceleration is set as a required decelerating torque. The electric motor and the engine of the hybrid electric vehicle are controlled so as to obtain this required decelerating torque. At this time, the electric motor operates as a generator to produce a regenerative braking force. The electric motor converts the regenerative braking energy into an electric power and a battery is charged with the converted electric power. In this way, energy recovery at the time of deceleration is effected. Such a required decelerating torque is set greater as the input speed of the transmission, i.e., the revolution speed of the electric motor is higher in order to obtain an adequate vehicle deceleration.
On the other hand, in the electric motor, an upper limit decelerating torque as an upper limit value of a producible regenerative braking torque is set based on specifications of the electric motor. This upper limit decelerating torque has a substantially fixed value in a low revolution speed zone, and is decreased along with an increase in the revolution speed of the electric motor in a high revolution speed zone based on characteristics of the electric motor.
Therefore, at the time of deceleration of a vehicle, there are two occasions, i.e., an occasion where the upper limit decelerating torque becomes equal to or greater than the required decelerating torque and an occasion where the upper limit decelerating torque is smaller than the required decelerating torque. Thus, when the upper limit decelerating torque is equal to or greater than the required decelerating torque, the following operation can be considered. That is, the clutch is disengaged, and the electric motor is controlled so as to obtain the required decelerating torque by using the regenerative braking torque of the electric motor alone, thereby effecting energy recovery during deceleration of the vehicle to a maximum extent. On the other hand, when the upper limit decelerating torque is smaller than the required decelerating torque, the following operation can be considered. That is, the clutch is engaged, and the electric motor and the engine are controlled so as to obtain the required decelerating torque by using both the regenerative braking torque of the electric motor and the decelerating torque of the engine.
Further, in regard to a driving torque when driving a vehicle, an upper limit driving torque as an upper limit of a producible driving torque is determined in the electric motor like the example of the decelerating torque. This upper limit driving torque also tends to be high in a low revolution speed zone of the electric motor and gradually decrease along with an increase in the revolution speed of the electric motor in a high revolution speed zone like the upper limit decelerating torque.
Therefore, at the time of, e.g., vehicle starting acceleration, there are two occasions, i.e., an occasion where the upper limit driving torque becomes equal to or greater than a required driving torque and an occasion where the upper limit driving torque becomes smaller than the required driving torque. Thus, when the upper limit driving torque is equal to or greater than the required driving torque that is necessary for driving a vehicle, the following operation can be considered. That is, the clutch is disengaged so that a driving torque of the electric motor alone can be used to obtain the required driving torque. On the other hand, when the required driving torque is greater than the upper limit driving torque, the following operation can be considered. That is, the clutch is connected so that both a driving torque of the electric motor and a driving torque of the engine can be used to obtain the required driving torque.
However, when a driver releases the accelerator pedal to aim at deceleration traveling on a downward sloping road, the upper limit decelerating torque may become greater than the required decelerating torque if the revolution speed of the electric motor is sufficiently low at the time of releasing the accelerator pedal. In this case, the clutch is disengaged, and regenerative braking of the electric motor alone is used to generate the required decelerating torque. Then, when a traveling speed is gradually increased since a gradient of the downward sloping road is steep, the revolution speed of the electric motor is also increased, and the required decelerating torque is consequently increased. On the other hand, the upper limit decelerating torque may be decreased. In such a case, when the upper limit decelerating torque is reduced below the required decelerating torque, the clutch is engaged. At this moment, the revolution speed of the electric motor may be greatly increased. When the clutch is engaged, the revolution speed of the engine jumps up from the idle speed before the engagement of the clutch.
In this manner, when the clutch is engaged in a state where the revolution speed of the electric motor is increased at the time of traveling down, e.g., a sloping road, the following problem occurs. That is, since the revolution speed of the engine that has calmly driven until this moment is suddenly increased, a driver misconceives that a vehicle has a problem or feels discomfort.
Even in driving a vehicle, when the required driving torque is low during driving at a relatively high traveling speed, the upper limit driving torque may exceed the required driving torque. In such a case, the clutch is disengaged, and the electric motor alone is used to effect driving. When the accelerator pedal is pressed in such a state and the required driving torque is thereby increased, the required driving torque exceeds the upper limit driving torque. Then, the clutch is engaged when the required driving torque becomes greater than the upper limit driving torque. In such a case, the revolution speed of the electric motor may be considerably increased due to high-speed traveling. When the clutch is engaged, the revolution speed of the engine jumps up from the idle speed that is a revolution speed before the engagement of the clutch.
Therefore, in such a case, the following problem occurs. That is, since the revolution speed of the engine that has calmly driven until this moment is suddenly increased, a driver may misconceive that the vehicle has a problem, or may feel discomfort.